The publication "Markt & Technik--Design & Elektronik" [Market and Technology--Design and Electronics], issue 26, dated Dec. 22, 1987, describes voltage-monitoring integrated circuits (ICs) for microprocessor systems. The described microprocessors are used to secure volatile data in the event of a voltage failure, in that the system is set to recognize a drop below a defined threshold voltage before it occurs, thus leaving enough time to secure the data in a non-volatile memory.
Further, it is known in the case of conventional programmable control systems, particularly in the case of small control systems, to supply the control system via a separate, external, direct-voltage power pack. The supply voltage provided by this external power pack is switched, within the control system to be supplied, to an internal circuit configuration which assures protection against reverse voltage on the one hand, and on the other hand, assures a backup supply of supply voltage for the internal power supply during short-duration failures of the external supply voltage. Small control systems of this kind are generally used for smaller control tasks, which must be implemented at the lowest cost possible. For this reason, usually one and the same external power pack is used for the external voltage supply and for driving the signal inputs of the control system. The external voltage supply is switched internally, within the control system to be supplied, to a circuit configuration of an internal power pack. This circuit configuration has at least one backup capacitor for the backup supply of supply voltage, so that voltage failures of the external power pack of short duration (of the order of magnitude of approx. 10-15 ms) can be bridged by the backup capacitor, thus allowing the control system to continue functioning unaffected. Only in response to longer voltage failures, during which the internal backup voltage also collapses, is the arithmetic-logic unit of the control system switched into a defined initial state by a "power-fail" signal (PFI). The disadvantage of this, however, is that the short-duration external voltage failures, which also occur at the signal inputs of the control system, can result in misinformation in an application program to be processed.